Moisture indicator for photoflash lamp

ABSTRACT

A photoflash lamp having a moisture indicator comprising a spot of dried paste disposed on the inside surface of the hermetically sealed lamp envelope to indicate by change in coloration the presence of air contamination. The paste comprises a mixture of cobaltous cobalticyanide and a binder of partially hydrolyzed polyvinyl alcohol.

United States Patent [191 Shaffer 3,770,362 Nov.6, 1973 [54] MOISTUREINDICATOR FOR PHOTOFLASH 3,195,326 7/1965 431/13 MacDonough et al..431/13 LAMP [75] Inventor: John W. Shaffer, Williamsport, Pa.

[73] GTE Sylvania Incorporated,

Danvers, Mass.

Dec. 23, 1971 Appl. No.: 211,450

l a e n e g r- C b d a 8 Primary Examiner-Carr0ll B. Dority, .lr.Att0rneyNorman J. OMalley et a1.

Assignee:

[22] Filed:

[57] ABSTRACT A photoflash lamp having a moisture indicator comprising aspot of dried paste disposed on the inside surface of the hermeticallysealed lamp envelope to indicate by change in coloration the presence ofair contamination. The paste comprises a mixture of cobaltouscobalticyanide and a binder of partially hydrolyzed polyvinyl alcohol.

2,993,008 7/1961 Anderson.............................. 431/13 6 Claims,2 Drawing Figures MOISTURE INDICATOR FOR PHOTOFLASl-I LAMP BACKGROUND OFTHE INVENTION This invention relates in general to the manufacture ofphotoflash lamps and similar devices having a sealed envelope, and. moreparticularly to the provision therein of means for determining thepresence of air contamination.

Photoflash lamps generate their actinic light output by the burning ofan energetic fuel, such as -finely shredded zirconium or aluminum metal,in a combustion supporting atmosphere such as oxygen. The containingvessel, or envelope, for such combustion must be transparent and must behermetically sealed so that a chosen atmospheric composition at a chosenstoichiometric balance will be present. Such lamp envelopes are mostcommonly made from glass. A crack or other defect in the envelopedestroys its hermeticity and the resulting air coamination adverselyaffects the light output and timing characteristics of the flashlamp.Accordingly, it is commonpractice in the manufacture of photoflash lampsto apply a spot of moisture indicating material to the inner surface ofthe envelope to indicate, by a color change, whether or not the hermeticclosure is intact. The material generally usedfor this purpose iscobaltous cobalticyanide, which in the anhydrous state is blue and whichbecomes pink when'hydrated. The water vapor normally present in aireffects a spot color change from blue to pink when the hermetic seal ofthe lamp envelope is broken.

Cobaltous cobalticyanide is a fine, water-insoluble powder. In order toattach a layer of this material to the inner surface of the glassflashlamp vessel it is necessary to use it in conjunction with a bindingagent. Presfrom one lot to another. Animal glue sometimes gels,

is subject to bacterial spoliage, and does not strongly adhere to glass.Shredded zirconium metalis abrasive like steel wool. Spots are sometimesabraded off of the glass wall during insertion of such shreds into thelamp. Salts such as barium or strontium chloride give soft, easilyabraded spots. The low viscosity of spotting fluids using salts asbinding agents permits rapid settling of the cobaltous cobalticyanide sothat continuous stirring must be used. Colloidal magnesium silicatebonds quite strongly to glass and gives abrasion resistant spots.However, the high degree of thixotropicity developed in such spottingfluids makes application of a thin layer nearly impossible in smalldiameter lamp vessels, with the undesirable result of a thick spotdeposit which is very slow drying.

Despite these recognized shortcomings, the use of some of these bindingagents with cobaltous cobalticyanide was quite extensive over the yearswith many of the commercial photoflash types because it was found thatwith relatively large lamp envelopes using aluminum as the combustiblematerial and gas fill pressures below atmospheric, many of thesedeficiencies were tolerable. Further, colloidal magnesium silicateproved to be a useful binding agent in the larger zirconiumfilled lampswithin the range of envelope sizes less than cc. volume. The relativelylarge envelope volume and manufacuring processes associated with theselamps rendered the thixotropicity problem of little consequence.

More recently, however, the trend in the flashlamp industry has been tothe use of subminiature lamps having envelope volumes of less than onecubic centimeter. Such subminiature lamps are presently mass produced inlarge quantities for use in the small photographic flashlamp unitsreferred to as flashcubes. In order to provide the desired light outputlevels, these lamps are densely packed with shredded zirconium foil andfilled with oxygen at pressures many times higher than atmospheric. Inthe presence of the significantly increased abrasive action ofzirconium, which is further aggravated by the dense packing of the foil,the deficiencies of animal glue and barium chloride are no longertolerable.

The high degree of thixotropicity developed in the spotting fluidscontaining colloidal magnesium silicate poses an especially severeproblem with respect to the manufacture of subminiature lamps. Firstly,it is very difficult to even apply the resultant thick paste on theinner surface of the subminiature envelope. The usual method of spotapplication is by use of a wire dip stick which is initially dipped intothe spotting fluid and then inserted into the glass envelope to deposita spot of the indicating paste on the inside surface thereof. With aspotting fluid containing a binder of colloidal magnesium silicate,however, the paste on the dip stick becomes so thick due to rapidevaporation that it becomes difficult, if not impossible to insert thepaste- Accordingly, spotting fluids based on colloidal magnesiumsilicateare' not used in subminiature all-glass envelopes.

SUMMARY OF THE INVENTION In view of the foregoing, it is an object ofthis invention to provide a superior moisture indicator for photoflashlamps.

Aparticular object of the invention is to provide an improved bindingagent for a photoflash lamp moisture indicator which adheres morestrongly to glass and which promotes much greater spot integrity andabrasion resistance, thereby resulting; in-a satisfactory and reliableair leak indicator for super-atmospheric flashlamps employing zirconiumas the combustible fill.

A principal object is to provide :an improved moisture indicatingmaterial for photoflash lamps which is compatible with the high speedautomated manufacture of subminiature flashlamps.

These and other objects, advantages and features are attained, inaccordance with the principles of this invention, by using partially.hydrolyzed grades of polyvinyl alcohol as the binding agent for moistureindicating spots in flashlamps. These resins have been found to provideexcellent spot adhesion and abrasion resistance and are available in aseries of viscosity ranges that per- BRIEF DESCRIPTION OF THE DRAWINGThis invention will be more fully described hereinafter inconjunctionwith the accompanying drawings, in

i which:

FIG. 1 is'an elevational view of an electrically ignitable photoflashlamp provided with a moisture indicator in accordance with'theprinciples of the invention,

and I FIG. 2 is a sectional elevation of a percussive-type photoflashlamp provided with a moisture indicator, according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENT The teachings of the presentinvention are applicable to either percussive or electrically ignitedphotoflash lamps of a wide variety of sizes and shapes; however, theinvention is particularly advantageous'asapplied to flashlamps havingenvelopes with'a volume of less than one cubic centimerer (cc.).Accordingly, FIGS. 1 and 2- respectively illustrate electrically ignitedand percussive type photoflash lamp embodying the principles of theinvention. I g

Referring to FIG. l,- the electricallyignitable photoflash lampillustrated therein comprises an hermetically sealed light-transmittinglamp envelope 2 of'glass tubing having a-press'4 defining one endthereof and an exhaust tip 6 defining the other end thereof. Supportedby the press 4 is an ignition means comprising a pair of lead-in wires 8and 10 extending through and sealed into the press. A filament 12, spansthe innerends of the lead-in wires, and beads of primer l4 and 16 arelocated on the inner ends of the lead-in wires 8 and 10 respectively attheir junction with the filament. Typically, the lamp envelope 2 has aninternal diameter of less than one-half inch, and an internal volume ofless than 1 cc. A quantity of filamentary combustible material 18 suchas shredded zirconium foil, is disposed within the lamp envelope. Theenvelope 2 is also provided with a filling of combustionsupporting gas,such as oxygen for example, 'at a pressure of several atmospheres.Preferably, the combustible and the combustion-supporting gas aresubstantially in stoichiometric balance.

The percussive-photoflash lamp illustrated in FIG. 2 comprises a lengthof glass tubing defining an hermetically sealed lamp envelope 22constricted at one end to define an exhaust tip 24 and shaped to definea seal 26 about a primer 28 at the other end thereof. The primer 28comprises a metal tube 30, a wire anvil 32 and charge of fulminatingmaterial 34. A combustible such as filamentary zirconium 36 and acombustionsupporting gas such as oxygen are disposed within the lampenvelope, as described for the electrically ignited lamp. The wire anvil32 is centered within the tube 30 and is held in place by acircumferential indenture 38 of the tube 30 which laps over the head 40or other suitable protuberance at the lower extremity of the wire anvil.Additional means, such as lobes 42 on wire anvil 32 for example, mayalso be used in stabilizing the wire anvil, supporting it substantiallycoaxial within the primer tube 30 and insuring clearance between thefulminating material 34 and the inside wall of the tube 30. A refractorybead 44 is fused to the wire anvil 32 just above the inner mouth of theprimer tube 30 to eliminate bum-through and function as a deflector todeflect and control the ejection of hot particles of fulminatingmaterial from the primer. The lamp of FIG. 2 is also typically asubminiature type having envelope dimensions similar to those describedwith respect to FIG. 1.

Although the lamp of FIG. 1 is electrically ignited, usually from abattery source, and the lamp of FIG. 2 is percussion-ignitable, thelamps are similar in that in each the ignition means is attached to oneend of the lamp envelope and disposed in operative relationship withrespectto the filamentary combustible material. More specifically, theigniter filament 12 of the flash lamp in FIG. 1 is incandescedelectrically by current passing through the metal filament support leads8 and 10, whereupon the incandesced filament ignites the beads of primerl4 and 16 which in turn ignite the combustible'18 disposed within thelamp envelope. Operation of the percussive-type lamp of FIG. 2 isinitiated by an impact onto tube 30 to cause defiagration of thefulminating material 34 up through the tube to ignite thecombustible 36disposed within the lamp envelope.

In the manufacture of photoflash lamps of the type just described, thelamp envelope 2 or 22 is initially a segment of glass tubing, open atboth ends. In the normal sequence of operations, the mount structurewhich comprises filament 12 and lead-in wires 8 and 10, or the primer28, is positioned in one of the open ends of the glass tubing'and thepress 4 or seal 26 is formed, thereby closing one of the open ends ofthe tubing. A suitable applicator is then inserted into the remainingopen end of the tubing to provide the inner wall thereof with a moistureindicator spot, denoted as 20in FIG. 1 and 21 in FIG. 2. Thereafter, thepaste spot 20, or 21, is dried, and a charge of combustible, in thisinstance shredded zirconium foil, is introduced into the remaining openend of the tubing. The glass is then constricted at the open end, andthe envelope is exhausted, filled with oxygen gas at several atmospherespressure, and tipped off at 6 to define a hermetically sealed envelope.Thereafter, the spot of dried paste 20, or 21, which normally has a blueappearance, functions to indicate, by a change in coloration to pink,the presence of an air leak in the envelope.

The photoflash lamp illustrated in FIG. 2 is of the type identifiedcommercially as an MOI and which is used in percussive-flashcubes of thetype described in US. Pat. 3,597,604. The lamp envelope volume of a MC-lis about 0.68 cc.; the gas pressure is about 550 cm. of Hg; and thequantity of shredded zirconium foil with which the lamp is provided isabout 29 mgs. per cc. of envelope volume.

In accordance with the present invention, the dried paste of themoixture indicator spot 21 comprises a mixture of cobaltouscobalticyanide and a binder of partially hydrolyzed polyvinyl alcohol.Upon use in thousands of lamps of the MC-l type described, thiscomposition has exhibited excellent spot adhesion and abrasionresistance characteristics. Further, as the partially hydrolyzed gradesof polyvinyl alcohol are commercially available in a series of viscosityranges, wide flexibility is permitted in the choice of solids contentand viscosity of the spotting fluid formulation prepared by stirring theaforementioned mixture in water. The polyvinyl alcohol solutions exhibitexcellent pigment I suspension characteristics and are non-thixotropic.Ac-

cordingly, the use of partially hydrolyzed polyvinyl alcohol as thebinding agent for the moisture indicator is readily adapted to the dipstick method of spot application, permits a controlled spot layerthickness to be applied, and thereby provides realistic dryingrates.

Polyvinyl alcohol is prepared by hydrolytic removal of acetyl groupsfrom polyvinyl acetate. Two types of polyvinyl alcohol are commerciallyavailable-mem pletely and partially hydrolyzed grades. The completelyhydrolyzed grades have 97 percent or more of the acetyl groups removed,whereas the partially hydrolyzed grades retain from percent to percentof the acetyl groups. e

The completely hydrolyzed grades of polyvinyl alcohol provide good spotadhesion and abrasion resistance, but the moisture'sensitivity of thecompletely dried spot is poorer. For that reason, previousinvestigations in search of a better binding agent for use in moistureindicators for photoflash lamps rejected polyvinyl alcohol as unsuitabletor the application. Consequently, it was with considerable surprisethat l discovered the superb suitability of the partially hydrolyzedgrades of polyvinyl alcohol as a binding agent for the moixtureindicating material; these particular grades of polyvinyl alcohol notonly provide the advantages outlined above, but when used in properquantities as a binding agent, the moisture indicating substancefunctions at a sensitivity comparable to that of the best prior artformulations.

Other materials evaluated as possible binding agents for moistureindicating spots included hydroxyethyl cellulose, hydroxypropylcellulose, carboxym'ethyl cellulose, poly(ethyleneoxide), and polyvinylpyrrolidone. None of these materials approach the degree of adhesion andabrasion resistance attained by polyvinyl alcohol. Some materials werenot sufficiently permeable to moisture to give good spot sensitivity.

By way of example, I have found that a highly satisfactory moistureindicator may be prepared by combining a mixture of about 10% ofpartially hydrolyzed polyvinyl alcohol and 90% cobaltous cobalticyanideby sired percent of solids. Viscosity is controlled by the grade ofpartially hydrolyzed polyvinyl alcohol resin chosen as well as therelative quantity of water used. A resin that gives a viscosity of about4 to 6 centipoise when diluted to about 4 percent by weight in water issatisfactory.

The partially hydrolyzed polyvinyl alcohol may comprise from about Ipercent to 20 percent of the dried spot by weight. At very low values,adhesion and spot integrity decreases, whereas at high binder contentsthe spot sensitivity falls off rapidly.

What I claim is:

l. A photoflash lamp comprising:

an hermetically sealed light-transmitting envelope;

a quantity of filamentary combustible material located within saidenvelope;

a combustion-supporting gas in said envelope;

ignition means disposed in said envelope in operative relationship withrespect to said combustible material;

and a moisture indicator comprising a spot of dried paste disposed insaid envelope to indicate by a change in coloration the presence of airtherein, said dried paste comprising a mixture of cobaltouscobalticyanide and a binder of partially hydrolyzed polyvinyl alcohol.

2. The photoflash lamp of clamp ll wherein said spot of dried pastecontains between about 1 percent to about 20 percent by weight of saidpartially hydrolyzed polyvinyl alcohol.

3. The photoflash lamp of claim l wherein said mixture comprises about90 percent of cobaltous cobalticyanide and about 10 percent of partiallyhydrolyzed polyvinyl alcohol by weight.

4,. The photoflash lamp of claim 1 wherein said partially hydrolyzedpolyvinyl alcohol is of a type which yields a viscosity of about 4 to 6centipoise when diluted to about 4 percent by weight in water.

5. The photoflash lamp of claim l wherein said envelope is glass, saidspot of dried paste is disposed on the inside surface of saidenvelope,and said combustible material comprises strands of shredded zirconiumfoil.

6. The photoflash lamp of claim 5 wherein the volume of said envelope isless than about one cubic centimeter, and said envelope is filled withoxygen gas at a pressure above atmospheric, and wherein the weight ofsaid filamentary material per unit of envelope volume is greater thanabout 25 milligrams per cubic centimeweight in an appropriate vehiclesuch as water. Suffiter.

cient water is stirred into the mixture to give the de-

2. The photoflash lamp of clamp 1 wherein said spot of dried pastecontains between about 1 percent to about 20 percent by weight of saidpartially hydrolyzed polyvinyl alcohol.
 3. The photoflash lamp of claim1 wherein said mixture comprises about 90 percent of cobaltouscobalticyanide and about 10 percent of partially hydrolyzed polyvinylalcohol by weight.
 4. The photoflash lamp of claim 1 wherein saidpartially hydrolyzed polyvinyl alcohol is of a type which yields aviscosity of about 4 to 6 centipoise when diluted to about 4 percent byweight in water.
 5. The photoflash lamp of claim 1 wherein said envelopeis glass, said spot of dried paste is disposed on the inside surface ofsaid envelope, and said combustible material comprises strands ofshredded zirconium foil.
 6. The photoflash lamp of claim 5 wherein thevolume of said envelope is less than about one cubic centimeter, andsaid envelope is filled with oxygen gas at a pressure above atmospheric,and wherein the weight of said filamentary material per unit of envelopevolume is greater than about 25 milligrams per cubic centimeter.